5. What is transformer? How many types of transformers?
Ans- A transformer is an electrical device that transfers electrical energy
between two or more circuits through electromagnetic induction. It
consists of primary and secondary windings wound around a magnetic
core. When alternating current (AC) flows through the primary winding,
it creates a varying magnetic flux in the core, which induces a voltage in
the secondary winding.
types of transformers
Core type: The primary and secondary coils are wound around a steel
core
Shell type: The primary and secondary coils are placed around a steel
core in a different way than in a core type transformer
6. Explain the basic structure of DC machine? What are the types of Dc
machine?
Ans- A DC machine has two main parts: a stator and a rotor. The stator is
stationary, while the rotor is free to move. The stator contains the
brushes, magnets, and housing, while the rotor contains the windings,
commutator, and output shaft.
Dc mahine has two basic structure
Stator-- A stator is a stationary part of a machine that produces a
magnetic field that interacts with a rotating component called the
rotor. Stators are found in electric motors, generators, sirens, and mud
motors.
Rotor-- A rotor is a rotating part of a machine that transmits
mechanical power or produces electrical power. Rotors are found in
many machines, including helicopters, electric motors, and turbines.
7. Explain the open circuit test and short circuit test of Single-Phase
Transformer?
Ans-- Open circuit test:-
An open circuit test on a single-phase transformer is an electrical test
where one winding (usually the high voltage side) is left open while the
other winding (low voltage side) is supplied with rated voltage, allowing
measurement of the no-load current and core losses (iron losses) within
the transformer, which are crucial for calculating its efficiency and
voltage regulation
Short circuit test:-
8. What is armature reaction? Describe the effects of armature reaction on
the operation of D.C. machines . How the armature reaction is minimized
Ans- Armature Reaction in DC Machines
Armature reaction refers to the impact of the magnetic field produced by
the current flowing in the armature winding of a DC machine (motor or
generator) on the main magnetic field produced by the field windings
Effects of Armature Reaction on DC Machines
1. Distortion of the Main Magnetic Field:- The armature reaction
causes the main magnetic field to become distorted. This results in an
uneven distribution of flux density under the poles.
2. Shift in Magnetic Neutral Axis (MNA):- The MNA, where no EMF
is induced in the armature conductors, is shifted from its original position
due to the armature's magnetic field.
3. Flux Weakening:- The armature flux opposes the main field flux in
some regions (demagnetizing effect) and strengthens it in others
(magnetizing effect).
Minimizing armature reaction:
Compensating windings: These are additional windings embedded in the
pole faces that produce a magnetic field opposing the armature reaction,
effectively neutralizing its effects.
Interpoles: These are small poles placed between the main poles,
connected in series with the armature, and designed to produce a
magnetic field that helps to maintain the neutral plane at the correct
position.
Strong main field: Increasing the strength of the main field can minimize
the relative influence of the armature reaction.
Proper brush placement: Positioning the brushes accurately along the
magnetic neutral axis can also help reduce sparking issues.
9. Discuss different methods of speed control of a dc motor?
Ans --armature voltage control (varying the voltage applied to the
armature), field flux control (changing the magnetic field strength by
adjusting the field current), and armature resistance control (adding
resistance in series with the armature circuit.
Armature Voltage Control:
This is the most common method, where the voltage supplied to the
armature is varied using a power converter like a thyristor or a
variable voltage source.
Increasing the armature voltage increases the motor speed, while
decreasing it reduces the speed.
Field Flux Control:
By adjusting the current flowing through the field windings, the
magnetic field strength is altered, which in turn affects the motor
speed.
Increasing the field current (stronger magnetic field) reduces the
speed, while decreasing it (weaker field) increases the speed.
Armature Resistance Control:
Adding a variable resistance in series with the armature circuit limits
the current flowing through the armature, thereby reducing the motor
speed.
15. What is commutation? Describe the effects of commutation on the
operation of D.C. machines?
Ans- Commutation, in the context of DC machines, refers to the process
of reversing the current direction in an armature coil as it passes under a
brush, effectively converting the alternating current induced in the
armature windings into a unidirectional direct current output, achieved
through the use of a commutator and brushes.
Effects of commutation on DC machine operation:
Sparkling:-If commutation is not ideal (i.e., if the current reversal
happens too slowly), sparking can occur at the brush-commutator
interface, causing wear and potential damage to the machine.
Torque ripple:-Poor commutation can lead to fluctuations in torque due to
uneven current distribution in the armature coils.
Efficiency reduction:-Excessive sparking can lead to energy losses,
reducing the overall efficiency of the DC machine.
Factors affecting commutation:
Brush design:-The material, size, and pressure of the brushes significantly
impact the quality of commutation.
Commutator design:-The number of commutator segments, their
insulation, and surface condition affect commutation.
Armature reaction:-The magnetic field produced by the armature current
can interfere with the main magnetic field, impacting commutation.
